Emma Joan Douglas Boland, Dan Jones , Andrew Meijers, Simon Josey, Gael Forget

The Southern Ocean (SO) is a crucial region for the global ocean uptake of heat and carbon. There are large uncertainties in the observations of fluxes of heat and carbon between the atmosphere and the ocean mixed layer, which leads to large uncertainties in the amount entering into the global overturning circulation. In order to better understand where and when fluxes of heat and momentum have the largest impact on near-surface heat content, we use an adjoint model to calculate the linear sensitivities of heat content in SO mode water formation regions to surface fluxes. We find that the heat content of these regions is, in all three basins, most sensitive to recent, local heat fluxes, and to non-local wind one to eight years previously. This is supported by the calculation of sensitivities to potential temperature changes at constant density, which reveal the sources of the mode water formation regions, and by sensitivities to potential temperature changes with varying density, which reveal dynamic links with boundary current regions, the Antarctic Circumpolar Current, and wave-like features. A series of forward perturbation experiments in the fully non-linear model confirm that the adjoint model can accurately predict linear changes in heat content of fixed volume mode water formation regions. These experiments also highlight that nonlinear effects can be of importance, depending on the time and region investigated, and that the contribution of volume changes to heat content changes can be as large as or larger than the contribution from temperature changes.